1. Technical Field of the Invention
The present invention generally relates to optical burst switching (“OBS”) networks. More particularly, and not by way of any limitation, the present invention is directed to a method of implementing partially preemptive burst scheduling in an OBS network.
2. Description of Related Art
In OBS technology, data bursts, each of which is made up of multiple packets, are switched optically at core nodes in the OBS network. A small control packet, called the Burst Header Packet (“BHP”) travels an offset time ahead of the data burst along the same data burst route and configures the optical switch for the duration of the data bursts at the core node. Basically, the BHP travels an offset time ahead of the data burst and establishes a path for the burst.
Because a BHP arrives at core nodes prior to its corresponding data burst trying to configure the switch for transparent fly-through, a short-term scheduling calendar, referred to as a “Sliding Frontier” or “SF”, must be maintained for prospective data burst arrivals. The SF includes all of the data channels and control channels for core node adjacency in the core OBS network. The SF progresses with the current time on the near-end and the latest scheduled burst on the far-end. All scheduled data bursts and their corresponding BHPs are maintained in the SF data structures such that each pair is doubly linked to each other.
FIG. 1 illustrates an SF 100 comprising k data channels D1-Dk and n control channels C1-Cn. As illustrated in FIG. 1, a first data burst 102a and its BHP 104a are scheduled on data channel Dk and control channel Cn, respectively. A second data burst 102b and its BHP 104b are scheduled on data channel D1 and control channel C1, respectively. A third data burst 102c and its BHP 104c are scheduled on data channel D2 and control channel C1, respectively.
Contention may occur between multiple data bursts attempting to traverse the same egress link. In current OBS technology, data bursts are scheduled intact.
Accordingly, when a burst overlaps another already-scheduled burst, it is dropped in its entirety, even if the overlap is minimal. This results in a drop in utilization. As will be defined herein, five different overlap situations may occur. FIG. 2A illustrates a case in which there is “No Overlap” between an already-scheduled burst 200 and a burst under consideration 202. In this case, there is no contention between the two burst 200, 202, and both can be scheduled without conflict. FIG. 2B illustrates a case of “Head Overlap”, in which case a front portion or “head” 212 of a burst under consideration 214 overlaps with a rear portion 216 of an already-scheduled burst 218. FIG. 2C illustrates a case of “Tail Overlap”, in which case a rear portion or “tail” 222 of a burst under consideration 224 overlaps with a front portion 226 of an already-scheduled burst 228. FIG. 2D illustrates a case of “Head-Tail Overlap”, in which case a head 232a and tail 232b of a burst under consideration 234 respectively overlap with a rear portion 236a of a first already-scheduled burst 238a and with a front portion 236b of an second already-scheduled burst 238b. FIG. 2E illustrates “Full Overlap”, in which case the entirety of a burst under consideration 244 overlaps with a portion 246 of an already-scheduled burst 248.
In normal practice of OBS technology, if any of the above-described types of overlap occur, the entire burst under consideration is dropped, even if only a small fraction thereof overlaps with an already-scheduled burst. This causes a fragmentation effect that results in an inefficient use of bandwidth because it leaves raw bandwidth unutilized. It also increases packet loss.
Possible solutions to inefficient bandwidth utilization, fragmentation, and high packet loss caused by the above-described practice include optical buffering and wavelength dimensioning. For quality of service (“QoS”) support in OBS, there is an approach that uses offset value adjustments. Unfortunately, optical buffering is not a matured technology and can be performed only in a limited way by Fiber Delay Lines (“FDLs”), which have a big footprint in equipment design and are a source of optical impairments. FDLs do not offer random access feature and can be designed only in segments. Further, optical buffering itself causes additional fragmentation. Wavelength dimensioning also has a physical limit in terms of number of wavelengths band requires full-scale wavelength conversion, which is not an easy practice. Offset value adjustments affects negatively the end-to-end delay and is not flexible enough; for example, it is not capable of accommodating multiple classes.